Microbiota of newborns: maternal bacteria have the upper hand

For the first time, researchers analyzed in detail the transmission of microbiomes from mother to newborn baby. Intestinal bacteria from the mother seem to be more prone to durably colonize the newborn’s oral and intestinal microbiotas.

Considering that the microbiome is shaped during the first months of life, an international team studied the impact of the mother’s microbiome on the newborn’s. To this end, the researchers monitored 25 mothers and their babies from the day following birth and made a follow-up at 3 days, one week, one month and 4 months. The intestinal, vaginal, oral and skin microbiotas of the mothers were analyzed while oral and fecal samples were collected from the babies.

Diversity peak followed by a quick selection

On their first day of life, newborns experience a massive influx of bacteria coming not only from the mother but also from the environment, making their microbiomes very diverse. On the third day, this diversity sharply decreases, before increasing again after the first week. A selection therefore occurs among original bacteria: some disappear while others remain (such as Bacteroides vulgatus or Bifidobacterium longum in the intestinal microbiota). This selection takes place in connection with a change in the environment: in the first days, the original strict anaerobic bacteria (in utero) give way to species that better tolerate oxygen and grow again after this first transition phase (oral and intestinal environment favor strict anaerobic species).

Maternal bacteria take root

The intestinal microbiota of the mothers proved to be a major contributor to transmitted strains, making up 22.1% of total intestinal bacteria of newborns on their first day, followed by vaginal (16.3%), oral (7.2%) and skin (5%) microbiotas. Six out of the 23 maternal strains transmitted from birth were found at the end of the study. This mother-to-child transmission seems to play a key role since the strains coming from the mother predominate among remaining bacteria: 70.5% of them were not replaced after four months, versus only 27% of strains coming from the environment. Similar results were observed in the oral microbiota: maternal oral bacteria make up most of the babies’ oral microbiota and then experience a selection process that favors them.

The maturation process still has to be explored

This study thus confirms the major role of the mother’s microbiome in the development of the baby’s. It also highlights the unsuspected diversity and heterogeneity of these fledging microbiomes. Although they show some degree of convergence, babies’ microbiomes are still very different from their mothers’ at the age of four months, therefore suggesting a longer maturation period than previously believed. The role of other microbiomes involved in this vertical transmission–that of the environment and that of the people who come in close contact with the newborns–still needs to be assessed.